I know this does not fit in with the "leftover" aspect of the thread, but I have heard you can make a really good dry rub using espresso coffee grounds, brown sugar and kosher salt. I plan on trying this soon!

I do grind the brown sugar, or sieve it or something to make it as fine as possible. The quantities are listed in order. The coffee and cocoa are almost equal, the cocoa, slightly less. Then the brown sugar amount is just below the cocoa amount.